Telecommunications technology is becoming increasingly digital in order to realize the economies made possible by progress in the field of large scale integrated digital circuits and in order to permit the integration of voice and data communications services in a common system. The technique for doing this most commonly involves the conversion of voice signals to pulse code modulated (PCM) form and the time division multiplexing of these PCM voice channels together with data channels. The switching of such digital, time division multiplexed (TDM) channels is accomplished in a digital TDM switch. Three stage switches are suitable for all but the smallest and a few of the largest exchanges.
Although switches of this type are well adapted to mixed voice and data traffic they also offer good functional characteristics for pure voice or pure data traffic.
Modular design of switches is highly desirable for economy of production, ease of installation, growth, and rapid field maintenance by personnel with limited training. The structural approach to satisfy this need thus far has been to consider the input section, the crosspoint matrix, and the output section as three separate entities, and to modularize each for field replacement or growth, as desired.
Progress in the art of large scale integrated circuits has made it possible to place thousands of gates or bits of memory on a single small chip of silicon.
A survey of some of the design considerations in the use of large scale integration is set forth in an article entitled, "Design Considerations for a Digital Switch" by M. Karnaugh, Proc. International Switching Symposium, Vol. 1, Kyoto, Japan, Oct. 25-29, 1976. By placing a number of such chips in a packaging module and a number of first level modules on a card or by using a fairly large first level module as a plug-in component to a circuit board, it is possible to construct a field replaceable module having very substantial functional complexity. This has advantages for field maintenance, because it is relatively easy to localize faults to large field replaceable units. Also, the number of distinct part types kept in inventory may be reduced if the parts are more complex in function. However, large modules require the development of complex hardware and software systems for testing in production and in factory maintenance. At the present state of the art, the cost of testing a product is roughly one third of the total cost of producing it. The increasing complexity of testing as integrated circuit technology advances is set forth in the article ATE: Not So Easy by Edward A. Toners, IEEE Spectrum, April 1977. In order to minimize the cost of test systems, reduce parts inventories, and simplify maintenance, it is desirable to assemble switches out of as few part types as possible.